LBC ASTROMETRY

The astrometricsolution is found by using a software suite developed by
M. Radovich at Naples Observatory. The steps through which the images are
astrometerized are:

1) match the image objects catalog with the astrometric reference catalog
(usually USNOB1.0). In order to increase the number and reliability of
matches, a standard presolution is applied before the initial matching.

2) find a preliminary solution chip by chip.

3) find a general solution for the whole mosaic at once.

4) write individual solutions for the four chips in the respective headers.

The astrometric headers are then saved as separate files (with extension
.head) that can be used later by a resampling program (Swarp, by E. Bertin),
or used to computed sky coordinates of objects measured on the images.

An astrometric header is shown below. From this file, the procedure to obtain
RA,DEC from x,y is the usual one: first, the x,y must be transformed into the
projected plane coordinates x',y' by applying the linear WCS
(keywords: CRVALn, CRPIXn, CDi_j).
Then, the coordinates are transformed into the native spherical coordinates
xi,eta by applying the astrometric polinomial solution PVi_j.
The meaning of the coefficients can be found in various documents over the
internet, for example here: http://www.oacn.inaf.it/~radovich/WIFIX/astrom.ps

The projected coordinates are then transformed into celestial ones by applying
the deprojection as indicated in the keywords CTYPEn and CUNITn, relative to
the tangent point on the celestial sphere CRVALn.

Example of astrometric solution header:

EXPTIME = 1.259

EPOCH = 2000.00000

EQUINOX = 2000.0

CRVAL1 = 102.98410

CRVAL2 = -0.34925

CROTA1 = -0.00177

CRPIX1 = -1179.12952

CRPIX2 = 2823.02173

CDELT1 = -6.389E-05

CDELT2 = 6.389E-05

CD1_1 = -6.325283e-05

CD1_2 = 4.880472e-07

CD2_1 = 5.258577e-07

CD2_2 = 6.325923e-05

IMAGEID = 1

INLIST = list.1.txt

KTAB = Y

LINXRMS = 0.210

LINYRMS = 0.246

LINCAT = USNO

LINTOL = 5

ASTXRMS = 0.121

ASTYRMS = 0.136

NAST = 439

PRESOL = ./Presol/ps1

PHFLAG = 1

OLXRMS = 0.036

OLYRMS = 0.025

PSORD = 3

PV1_0 = 0.0001547168

PV1_1 = 1.0032149315

PV1_2 = 0.0009429324

PV1_4 = 0.0240833660

PV1_5 = 0.0166469825

PV1_6 = 0.0005309086

PV1_7 = -0.1984784774

PV1_8 = 0.0469841415

PV1_9 = -0.2529869539

PV1_10 = 0.0030073688

PV2_0 = -0.0000049588

PV2_1 = 1.0003792280

PV2_2 = 0.0012891438

PV2_4 = 0.0086788762

PV2_5 = 0.0053759130

PV2_6 = 0.0116861035

PV2_7 = -0.2621941903

PV2_8 = 0.0159960531

PV2_9 = -0.2434173057

PV2_10 = 0.0214992420

ZP = 0.250

FLXSCALE= 7.942812e-01

ZP0 = 0.000

END

UCAC2 2230+35

Here it is possible to download some
exposure of the astrometric field UCAC2 2230+35 taken in B, U and
V filters. Original MEF images have been pre-reduced (x-talk
corrected, bias subtracted and flat-fielded) and splitted in
single FITS images. Each tar file contains 40 pre-reduced frames
with the corresponding weight image (.wht.fits file) and
astrometric solution (.head file). The .head files can be
immediately used with the SWarp
program to obtain aligned images.